Cellular standards often specify physical channels that provide different radio link rates as units of quasi-static resource allocation for both circuit and packet mode services. For example, Full Rate (FR), Half Rate (HR) and Quarter Rate (QR) channels that provide radio link rates in the ratio 1:0.5:0.25, respectively, may be specified.
For example, GSM standards specify an adaptive multi-rate (AMR) speech codec with 8 distinct modes ranging from 4.75 to 12.2 kbps. AMR speech frames may be carried over full rate (FR) or half rate (HR) speech traffic channels that provide a radio link of 22.8 kbps and 11.4 kbps, respectively. With AMR, a GSM system may choose the combination of speech codec modes and channel rates to achieve the required quality of service with minimum system and radio resources. On the assigned traffic channel, AMR codec mode adaptation changes the level of source and channel coding to achieve the best speech quality under the prevailing channel conditions. In such cases where different physical channel rate options are provided, the system capacity and/or spectral efficiency may significantly vary depending on the frequency reuse employed, the mechanisms used for selecting these channels and switching or adapting between these channels.
Previous studies of half and full rate channel adaptation focused on the tradeoff between the grade of service (GOS) and system capacity. Allocating more half rate or quarter rate channels increases the total number of calls a system can admit. However, lower rate channels provide less protection than full rate channels for the same vocoder mode under poor channel conditions. Furthermore, switching between FR, HR and QR channels is treated as an intra-sector handover, and frequent switching may degrade speech quality on account of speech frame blanking that occurs during each handover.
Allocation and packing of half- and full rate channels have been investigated assuming the system is limited by channel availability (i.e., hard blocking) and not interference. Re-packing of half rate calls is shown to provide capacity benefits when the fraction of half rate users is large. However, speech frame blanking associated with re-packing and its impact on GOS have not been considered. Furthermore, hard blocking assumptions are not appropriate for tight reuse (i.e., 1/3 or 1/1) deployments.
After a channel rate has been selected, the network must select a physical channel for assignment or packing. If only one class of channel rates is available, assignment among available physical channels are typically performed randomly. In the presence of multiple classes of channel rates, however, this method of assignment may lead to an inefficient allocation of resources and may increase blocking rates.